Put your mindset into "starting a business" frame. And you're starting a commercial cleaning business (even though what your cleaning are structurally homes and not stores or manufacturing plants).
The Small Business Administration has plenty of free information on how to start a small business. In the cleaning area, Building Service Contractors Association International ( http://www.bscai.org ) has plenty of information on how to start and operate a commercial janitorial and custodial business. (Disclaimer: I used to work for BSCAI and wrote or edited many of their books on the subject.)
The keys to success are marketing, pricing, labor management, and business management. The work itself is fairly easy. But you need to market properly to get the jobs. You need to price it right--and every company's price structure is different. You need to handle your employees or independent contractors. (Many companies find that the most difficult part, with pricing a close second.) And you need to handle it like any business--decent financials, knowing your costs, keeping good records, and so on.
The question you posed indicates that you need to learn more about the industry. My advice: Get a job with a cleaning company. See how it actually operates from the inside. See what works and what doesn't. You don't have to be there a long time--6-9 months will give you a lot of the experience necessary. Then, if you wish, go off on your own. But it'll be the best 6-9 month education you can get--and you'll actually get paid for doing it.
Hope that helps.
Cassandra Black, Consultant/Author, CEO, Foreclosure Cleanup, LLC
Also Don's response is excellent. You should also check into the requirements for the disposal of hazardous items: flourscent bulbs, oil, paint, car batteries, chemicals, CRT type monitors, etc.
Generally, you are paid for your services after you have completed the job. You would be responsible for providing your own cleaning supplies and the cost for the removal of trash. These expenses should be figured into your estimate for doing the clean out and approved by the owner prior to doing the work.